This is a new application requesting five years of funding for studies aimed at understanding the pathogenesis of neurofibrillary tangles, one of the major histopathological markers of Alzheimer's disease. Neurofibrillary tangles are composed of bundles of paired helical filaments (PHF), whose main constituents have been identified as tau proteins and ubiquitin. The tau proteins incorporated into PHF are three polypeptides of molecular weight 68-60 kDa, which have been demonstrated to be abnormally phosphorylated forms of tau, also referred to as A68 or PHF-tau. Two populations of PHF that differ in solubility with sodium dodecyl sulphate (SDS) have been observed. One population of PHF, soluble in SDS, is composed primarily of PHF-tau. Components of the other population of PHF besides tau and ubiquitin have not been fully-identified, due to their insolubility in SDS. The structural and biochemical relationship between these two populations of PHF is unknown and it is objective of our current studies. In our project, we propose to examine the possible role of phosphorylation, ubiquitination and partial proteolytic digestion as events leading to an impaired solubility of PHF and the subsequent accumulation of PHF in neurofibrillary tangles. We will test the effects of in vitro ubiquitination and the specific hydrolysis of ubiquitin on several properties of PHF including their solubility. We will also investigate the effects of aluminum as a possible catalyst in the 'impairment of the PHF solubility and in the formation of PHF aggregates. The specific aims of this project include comparative structural studies of the two populations of PHF with an emphasis on their mass and physical dimensions as well as studies which will explore the incorporation of soluble PHF into neurofibrillary structures.